Optical sources and sensors are included in a variety of optical imaging systems. In a typical optical imaging system, an optical source including one or more LEDs or other emitters illuminates a target, such as an imaging surface or navigation surface. The sensor detects reflected, scattered or transmitted light from the illuminated target. In an optical imaging system used for navigation, outputs from the sensor are processed to extract position, velocity, acceleration, or other motion parameters of the optical imaging system relative to the target. In other applications, the output from the sensor is processed to characterize features of the illuminated target.
FIG. 1 shows an exemplary control signal C for timing image acquisitions in a conventional optical imaging system (shown in FIG. 2). In this example, the optical source within the optical imaging system is turned “on” by a falling edge transition in the control signal C and turned “off” by a rising edge transition in the control signal. Image acquisitions are triggered to occur at the same falling edge transition that turns the optical source “on”.
When the optical source is “off”, the target is not illuminated and the sensor does not receive light. During this “off” time the sensor will typically discharge due to current leakage (referred to as “dark current”) inherent within the devices used to implement the sensor, which can affect the sensitivity or transfer characteristics of the sensor.
In optical imaging systems where the sensor includes one or more CMOS detectors, photodiodes or other transducers, non-uniform discharge between these devices can result in non-uniform image sensitivity or artifacts in the images acquired when the sensor initially receives light from re-illumination of the target. In optical imaging systems used for navigation, dark current discharge of the sensor can result in cursor jump upon re-illumination of the target.
One approach avoids the dark current discharge of the sensor by illuminating the target continuously, so that the sensor continuously receives light. This approach has the obvious disadvantage of high power consumption, as constant illumination of the target translates to the optical source being on continuously. In a portable optical imaging system, such as an optical mouse for a computer, this high power consumption can lead to unacceptably low battery life. Accordingly, there is a need for an alternative way to accommodate for dark current discharge in the sensor within an optical imaging system.